Is 325 mg of Benadryl at 115 pounds dangerous? I have severe ptsd and take around 150/275 mg to sleep. Tonight I feel nauseous and have a headache

Yes considerother Rx. Yes - you are getting up there to maximum dosage especially with your weight. I hope you are addressing your PTSD with therapy and perhaps some other meds.
PTSD. I am assuming that this is an over the counter medication, since physicians have many other medications to chose from to help sleep (without going to these off-label doses). Please discuss this with your physician/ psychiatrist as alternative for sleep may be discussed.